The objective of the proposed research is to understand the mechanism by which vitamins are distributed to different tissues with egg yolk as the model system and biotin as the specific example. The working hypothesis is that the transport of vitamins is mediated by specific vitamin-binding glycoproteins synthesized in the liver, transported by the blood stream and taken up by tissues which recognize specific structural features (e.g., attached carbohydrate) of the vitamin-binding protein. A recently rediscovered biotin-binding protein in egg yolk will be isolated by affinity chromatography and characterized with respect to size, amino acid composition, carbohydrate content and biotin-binding properties. These properties will be compared to those of egg white avidin. The amount of biotin and biotin-binding protein in eggs will be monitored as a function of biotin availability in laying hens. Chemical modifications of the biotin-binding protein and their effect on biotin transport will be measured.